In commonly assigned U.S. Pat. No. 4,639,756, issued Jan. 27, 1987 and entitled "Graded Gap Inversion Layer Photodiode Array" J. P. Rosbeck and I. Kosai disclose a mesa-type photodiode array having U-shaped channels formed through an n-type capping layer, a p-type radiation absorbing base layer and partially into an underlying, more heavily doped, p-type buffer layer. The mesa structures are covered with a passivation layer having a fixed positive charge that inverts the underlying surface of the p-type base layer but not the more heavily doped buffer layer.
It is known that responsivity variations between individual photodetectors of such arrays are caused in part by variations in effective photodetector area. Variations of as little as plus or minus one micron may have a significant impact on response uniformity and, hence, may become a limiting factor in the usefulness of the array for certain applications.
This problem is especially apparent with photodetectors that are delineated by conventional mesa etching techniques. The inherently imprecise nature of the chemical etching process used to delineate the individual mesas and, hence, also the individual p-n junctions of the photodetectors typically results in the formation of ill-defined boundaries between adjacent photodetectors. Small variations in the etched structures result in variations in the effective optically sensitive areas of the photodetectors. One effect of the geometrical variation between mesa structures is that photocarriers created in a zone between two mesas may diffuse preferentially to either one of the p-n junctions contained within the two adjacent mesa structures. This results in the creation of non-uniformities in electrical output between the photodetectors.
By example, for a photovoltaic mercury-cadmium-telluride (HgCdTe) diode array, having a photosensitive layer that is 10 microns thick or greater, the geometric uniformity achieved through the conventional mesa etching process may exhibit an approximately one micron standard deviation. However, for the competing material platinum-silicide (PtSi), having only a 20 Angstrom thick photosensitive layer, the geometrical uniformity may exhibit only an approximately 0.1 micron to 0.2 micron standard deviation. This significant difference requires additional effort to correct for the larger non-uniformity that results from the use of the conventional mesa etch technique with HgCdTe.
The non-uniformity effect becomes more pronounced as the mesa structures are made smaller, thereby also reducing the effective optical area of each photodetector. In that it is often desirable to reduce the size of the mesa structures in order to increase the packing density and increase the optical resolution of the array, it can be appreciated that the induced photodetector non-uniformity may become a limiting factor that prevents the fabrication of a useable array below some minimum mesa structure size.
Although mesa etch processing techniques may eventually be improved, what is required, and what is thus one object of the invention to provide, is a method to precisely and uniformly delineate the optical areas of photovoltaic (PV) diodes in a mesa-type imaging array.
A further object of the invention is to provide an array of PV diodes, in a mesa-type imaging array, that exhibits a uniformity of diode response to incident electromagnetic radiation while also exhibiting a minimal degree of cross-talk between adjacent diodes.
Surrounding guard p-n junction have been previously employed to improve optical response uniformity. However, such guard junctions function by collecting rather than deflecting stray charge carriers. Thus, these conventional guard junctions act to reduce the effective optical area. This reduction in optical area is especially detrimental for densely-packed radiation detecting arrays.
It is thus an additional object of the invention is to provide improved detector uniformity without detrimentally impacting optical fill factor.
A still further object of the invention is to provide an array of PV diodes each of which is contained within an associated mesa structure, the array including integrally formed barriers between mesa structures to minimize the diffusion of charge carriers from a region associated with an optical collection area of one mesa structure to an adjacent mesa structure.